The present invention generally relates to the field of lawn care, grass and weed cutting apparatus and methods and more particularly, is directed to a flexible but rigid aerodynamic cutting string for use with such devices and methods as flexible line trimmers and the like.
Heretofore, there have been many cutting methods and devices for maintaining well-manicured lawns and grass areas, particularly adjacent buildings, fence lines and other obstacles protruding from the turf above the grass to be cut. Maintaining grass and weed areas along roadways, bankments and generally uneven surfaces also present particular problems to those responsible for their maintenance.
Present-day grass and weed cutting tools have not been without difficulties in that they are laborious and time consuming to use. Such tools include manually operated shear disk and scissor type devices mounted on extended handles. In other prior art cutting tools, a gasoline engine or electric motor is assembled to a wheeled frame and is used to drive a rotating blade in a vertical or inclined plane for cutting grass and weeds, particularly along the sides of buildings. Such a tool is expensive and can be dangerous to operate.
The availability of small electric motors and gasoline engines have lead to the development of improved grass cutting tools. Many of the tools take the form of a high speed rotating metal blade or flexible cutting line mounted on an extended handle. Conventional flexible line trimmers employ the use of flexible cutting line intended to cut grass, weeds, and vines in areas not intended or not possible for such devices as the walk-behind or ride-on lawn mower.
Through years of development, Original Equipment Manufacturers have developed flexible line cutters into one of the most widely used tools for grass cutting and care. Prior art flexible cutting line for flexible line trimmers include several attempts at providing a means to more efficiently, and cleanly, cut grass and weeds by providing generally cylindrical, round, star, triangle, and indented square shaped cutting line, none of which employ the concept of aerodynamic maximization, or attempt to control its longer axis into the plane of the cut.
The wetted surface area facing the air flow and total contour plays an important role in how air flows over a rotating cutting line. As the string is rotated on currently designed trimmers, a laminar flow is created, with a disruptive zone at the trailing side of the cutting edge of the line. The disruptive zone includes a pressure differential, and further creates a wake which leaves behind eddy disturbances for the next string to flow through. The pressure differential, in essence, pulls the string backwards.
The engine attempts to propel the string forward but the pressure differential negates a portion of the energy due to the creation of a separating type air foil from its boundary. Further, the eddy's and turbulent and/or laminar conditions created by the wake trailing the cutting line add additional forces to the forward entry path of the next forward moving line.
According to the invention it has been found that air drag and subsequent flow conditions play an important role in cutting efficiency and energy consumption in line cutting devices. Higher air drag and disturbed flow conditions leads to lower tip speed, lower fuel efficiency, and erratic line bending because more power has to be exerted in order to maintain proper and effective cutting speed. Air drag increases as the line size (diameter) increases from 0.050" to 0.130", and higher, but increases even more dramatically when sharper shapes such as square, star and cross lines are used. Therefore, as size increases and shape changes to more square or sharper cutting lines or more acute angular changes, the fuel efficiency further decreases, and the fuel usage increases. In contrast, by reducing the air drag on the string, speed will increase, as will energy efficiency as well as reduced environmental emissions.
The cutting elements and methods of the invention, aside from aerodynamic improvements, create additional benefits in reducing wear (a longer wearing surface with increased cross-sectional area compared to conventional line), increasing stiffness (a longer, more structural beam in the operating direction which increases the moment of inertia (I) of the formula-Stress=Moment*c/l), and creating a sharper cutting surface than circular shaped cutting line.
According to one aspect of the present invention an elongated flexible cutting string connectable to a rotating device for cutting vegetation is provided. The flexible cutting string has an aerodynamic cross-sectional configuration over at least a significant length (typically more than 80% of the length) thereof, including a maximum cross-sectional dimension and a minimum cross-sectional dimension typically less than 85% of the maximum cross-sectional dimension, and the configuration having a drag coefficient which is at least 5% less (preferably at least 10-300% less, e.g. about 50-200% less) than the drag coefficient of a string having a substantially round and smooth cross-section. For example, the string of the invention has a C.sub.D (drag coefficient) of 1.0 or less, e.g. 0.8 or less, preferably 0.6 or less, and most preferably about 0.35 or less.
The cross-sectional configuration may comprise a wide variety of shapes, perhaps the most simple of which is a simulation of the tear drop. The tear drop may have a textured (i.e. non-smooth on a macroscopic level) exterior surface, such as aerodynamic dimples (such as in a golf ball), a saw-tooth configuration, or an exterior surface wavy configuration. Other cross-sectional configurations include an ellipse with a ratio of at least 1.25:1 (preferably at least about 2:1, e.g. about 8:1), simulating a spearhead (comprising a small cross-sectional area rectangle merging into a large cross-sectional area trapezoid), simulating a keyway (having a small diameter circle merging into a large diameter circle, or having a small cross-sectional area trapezoid merging into a larger cross-sectional area trapezoid of substantially the same shape), simulating a diamond with rounded apices, or simulating an ice cream cone (e.g. having a small cross-sectional dimension substantially a cone-shaped portion merging into a larger cross-sectional dimension substantially parabola shaped portion). The elongated flexible cutting string preferably is primarily (that is at least 50.1%) or substantially (i.e. at least 90%) non-metallic, e.g. polymeric plastic, such as nylon, or other conventional reinforced polymeric plastics for such purpose, or rubber or rubber-like materials, or both plastic and rubber-like materials, and may have reinforcing fibers, or fillers, or other materials therein.
According to another aspect of the present invention a cutting element for cutting vegetation, which may be in the form of a flexible string or a substantially self-supporting (that is semi-flexible) strut, is provided comprising: An elongated substantially solid body comprising more than 50% polymeric material, and having a generally tear drop shaped cross-sectional area, and an exterior surface. The exterior surface may be textured, such as having aerodynamic dimples formed over the majority thereof, or having a wavy configuration. By using such "textures" the flow boundary layer is kept closer to the cutting element than if it were smooth, and separates more on the rear side of the cutting element instead of more toward the top or front. Therefore the pressure differential (perhaps even negative pressure at the rear) that exists from the front to the rear of the cutting element in the direction of rotation will be lower, thus creating a lower drag force. This occurs at the same Reynolds number. The dimpled (or other textured) configuration of the cutting element according to the invention will have a lower coefficient of drag (at least about 10% lower, preferably about 20% or more lower) because the dimples (or other texturing) change the separation flow to a point further to the rear of the profile.
While not nearly as advantageous as the strut configurations according to the invention, the golf ball simulating dimples may also be utilized in a conventional substantially circular cross-section cutting string. That is according to another aspect of the present invention there is provided a primarily polymeric plastic elongated cutting element having a substantially circular cross-section and an exterior surface, and golf ball simulating dimples formed over the majority (or substantially all) of the exterior surface.
According to another aspect of the present invention a method of cutting vegetation using at least one elongated flexible string having a beam-like structure with a first axis which is the strongest axis and has the highest moment of inertia, and a second, weaker, axis, is provided. The method comprises the steps of: (a) Rotating the elongated string about an axis of rotation so that the first axis of the beam-like structure thereof is positively mounted or controlled (e.g. by an eyelet with the same cross section as the string, by the natural forces acting on such a high inertia element, and/or by a compound taper of the string with longest cross-section at the rotating head, etc.) moves in a cutting plane. And, (b) bringing the rotating string into contact with vegetation so that the cutting plane is generally transverse to a portion of the vegetation to be cut, and the string cuts the vegetation.
The string may have an exterior surface with aerodynamic surface texturing, in which case step (a) is practiced to rotate the string so that the drag coefficient thereof is at least 10% less (preferably at least about 20% less) than if step (a) were practiced with an identical string having no surface texturing.
Step (a) may be practiced so that the drag coefficient of a single string is 1.0 (preferably 0.8, or 0.6, or 0.35, or less). For example, step (a) may be practiced on a theoretical or calculated basis by rotating a single string having a dimension perpendicular to the direction of movement of about 0.08 inches so as to use less than the equivalent of 200 wafts of power at about 9000 rpm for a 14 inch swath and 3.5 inch rotating head diameter; or by rotating a single string having a dimension perpendicular to the direction of movement of about 0.08 inches so as to use less than the equivalent of 0.4 horsepower at about 8000 rpm for a 17 inch swath and 4.5 inch rotating head diameter, or by rotating a plurality of strings each having a dimension perpendicular to the direction of movement of about 0.08 inches so as to use less than the equivalent of 0.4 horsepower at about 8000 rpm for a 17 inch swath, and 4.5 inch rotating head diameter, per string. The equivalents can be calculated, at least approximately, utilizing the following equations:
Drag=1/2 .rho. AC.sub.D V.sup.2 PA1 HP=Drag.cndot.V/550 PA1 C.sub.D =coefficient of drag PA1 .rho.=density of air PA1 V=velocity=w.cndot.r PA1 Area=D.cndot..DELTA.r PA1 D=diameter (largest cross-sectional dimension) of line; and PA1 .DELTA.r=radius extending from trimmer head. PA1 (a) Rotating the one or more primarily or substantially non-metallic flexible elongated strings having a dimension perpendicular to the direction of movement of about 0.08 inches about a common axis of rotation so that the aerodynamic profile of each moves in a common cutting plane so as to use less than the theoretical equivalent of 0.4 horsepower at about 8000 rpm for a 17 inch swath, 4.5 inch diameter rotating head per string. And, (b) bringing the one or more rotating primarily or substantially non-metallic flexible strings into contact with vegetation so that the common cutting plane is generally transverse to a portion of the vegetation to be cut, and the string cuts the vegetation. For example step (a) may be practiced by providing a rotating head, with an eyelet for each string through which the string extends, the eyelet having substantially an identical cross-sectional configuration to the string; and by powering the rotating head with a power source so that the head rotates, at least when the one or more strings are not in contact with vegetation, at the equivalent of between about 6000-9000 rpm for a 17 inch swath.
and
Where:
Therefore HP=1/2 .rho. A C.sub.D V.sup.3 /550
or ##EQU1##
For a typical substantially tear drop shaped strut, where the length is four times the thickness, the maximum coefficient of drag is estimated about 0.35 while in laminar flow conditions, whereas for a (macroscopically) smooth surfaced circle C.sub.D =about 1.2 (the lowest conventional string C.sub.D). If the diameter equals 0.095 inches and r equals eight inches, then the theoretical horsepower required to power the circular cross-section string is 0.3175 whereas the horsepower to power the tear drop shaped string is 0.0926.
Where step (a) is practiced by rotating a generally tear drop cross-sectional string, the tear drop cross-section can be rotated in one of two ways. As the tear drop cross-section string has a larger more rounded leading end, and a smaller more pointed trailing end, step (a) may be practiced by rotating the string so that the larger more rounded end provides the cutting edge of the string so that the larger more rounded end comes into first contact with the vegetation, or step (a) may be practiced by rotating the string so that the smaller more pointed end provides the cutting edge of the string and therefore first comes in contact with the vegetation. Or, the at least one string may comprise a plurality of flexible strings each having a beam-like structure with first and second axes, and step (a) may be practiced to rotate all of the plurality of strings so that the first axis of the beam-like structure of each is positively substantial and will move in substantially the same cutting plane. For example the plurality of strings may comprise at least three strings, e.g. four strings, so that step (a) is practiced to rotate the at least these strings with the first axis of each positively maintained in substantially the same cutting plane.
Because less power is necessary in order to move a same dimensioned string at a given RPM according to the invention, it is possible to either increase the number of flexible strings that can be powered by conventional engines, or increase the cutting diameter of the lines, or use smaller and lighter weight engines in the future, or use more portable battery powered or electric units, or convert conventional two cycle engines to less polluting two or four cycle engines. Also various modifications of each of these may be employed. For example the plurality of strings may comprise at least three strings, with step (a) practiced to rotate the at least three strings with the first axis of each in substantially the same cutting plane using a conventional approximately twenty-five cc gasoline powered two cycle engine, or a comparable four cycle engine.
Step (a) may be practiced by providing a rotating head, with an eyelet through which the string extends, the eyelet having a substantially identical cross-sectional configuration to the string, and powered by powering the rotating head with a power source. The rotating head may be powered by a battery or electric powered power source easily portable and usable by an average adult human.
According to another aspect of the present invention a method of cutting vegetation using one or more elongated or substantially non-metallic (e.g. primarily polymeric) flexible strings having an aerodynamic profile in a cutting plane, is provided. The method comprises the steps of:
According to the present invention cutting strings which have improved aerodynamic operation over such strings known in the prior art are provided. Cutting strings according to the present invention also can have improved durability by reducing wear over such strings known in the prior art, are easy to use, and low in cost, and have reduced cost of operation as a result of energy or fuel savings compared to conventional strings, and are easy to manufacture.
The invention includes a number of embodiments which eliminate or greatly reduce the drag, fluttering and force-imbalances which are created as a result of the pressure differential and wake which are formed behind the flexible line as it moves. This is achieved by employing an aerodynamic profile for the line which lowers the coefficient of drag and reduces the forces created by the air foil, or by producing conventionally shaped strings with drag-reducing texturing. The invention also encompasses related components, such as the rotating head, the eyelet and string spool.
The present invention can be used with continuous length (extruded) or fixed length (injection molded) flexible lines, formed of aerodynamic connecting links, pre-cut lengths of flexible line, and/or other moving, less flexible, cutting attachments. In addition, the aerodynamic cutting string of the present invention can be employed with single, double, triple, quadruple or multiple line heads of any given diameter.
It is the primary object of the present invention to provide a more efficient cutting string or other cutting element for a rotating head trimmer or the like, and a highly efficient method of cutting vegetation using such a string. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.